Organic light emitting diode (OLED) displays are already established in the market as an alternative to the yet dominating liquid crystal displays (LCDs), in particular for mobile applications, such as smartphones. A drawback of LCDs is that about two third of the light emitted from the backlight unit is absorbed by the red, green and blue areas of the color filter array. Because in OLED displays the desired red, green and blue light may be directly generated in corresponding color subpixels, no light has to be filtered out for tailoring the spectral emission curve. Together with the continuously improving luminous efficiency of active OLED materials, OLED displays may eventually be brighter than LCDs, which could be a decisive advantage, in particular for outdoor applications, where the light emitted from the display has to compete with the bright sunlight. Unfortunately, reflection of ambient light at the metallic anode layer of an OLED display reduces contrast and hence readability. To reduce the light reflection, OLED displays are equipped with a circular polarizer, which converts incident ambient light into circularly polarized light, which upon reflection at the metallic anode layer is then absorbed by the circular polarizer. Typically, the circular polarizer comprises a linear polarizer and a quarter wave plate, wherein the slow axis of the quarter wave plate is at 45° with respect to the absorption axis of the linear polarizer. Circular polarizers, in which a quarter wave retarder foil is laminated on a polarizer foil are commercially available since many years. Such circular polarizers may be applied on top of OLED devices.
Because of the high sensitivity of active OLED materials to moisture and oxygen, OLED devices have to be properly encapsulated. Glass encapsulation is very effective but is fragile, increases weight and thickness and does not provide high mechanical flexibility. Therefore, thin film encapsulation comprising a barrier stack including one or more barrier layers is desired. Typically, a barrier stack comprises at least one organic and one inorganic layer.
US'2013/0032830A1 discloses a polarizing foil comprising a wave plate, a linear polarizer plate and one or two barrier stacks. The foil incorporating circular polarization and barrier function is attached to a substrate comprising an OLED matrix by interposing an adhesive layer between the polarizing foil and the OLED substrate. Because of the desired optical function, the quarter wave plate has to be between the OLED device and the linear polarizing plate. A barrier stack may be either between the OLED substrate and the quarter wave plate or on the opposite side of the linear polarizing plate with respect to the quarter wave plate. The foil may also comprise two barrier stacks, one of them between OLED substrate and quarter wave plate and the other between quarter wave plate and linear polarizing plate. The polarizing plate is a standard PVA polarizer sheet comprising a PVA layer between two TAC (triacetyl cellulose) films. The quarter wave plate is a flexible foil compatible with roll to roll manufacturing.
In standard roll to roll manufacturing processes of polarizer and quarter wave retarder films both the absorption axis of the polarizer and the slow axis of the retarder film are either longitudinal or transversal to the web moving direction. Therefore, manufacturing of a circular polarizer, which requires an angle of 45° between the directions of the slow axis and of the polarization axis, does not allow to laminate both films in a simple roll to roll process, but requires additional cutting and aligning steps.
Optical retarder films can also be realized by coating a layer of liquid crystalline monomers on a substrate with a surface, which is able to orient liquid crystals. After the liquid crystal monomers are oriented, they may be polymerized or cross-linked in order to solidify the material. Such layers are also known as liquid crystal polymer (LCP) layers. U.S. Pat. No. 6,717,644 discloses cross-linkable liquid crystals, oriented by a thin layer of a photo-alignment material on a substrate.
Because of the demand for thin and lightweight mobile devices as well as for high throughput in production, there is a need for encapsulation structures for OLED displays, which have antireflection properties but are thinner and less complex in production, than what is available from the state of the art.